The brains of people with large numbers of Facebook friends are different from those of people with fewer online connections, say neuroscientists.

The researchers at University College London found that users with the greatest number of friends on the social networking site had more grey matter in brain regions linked to social skills. The finding suggests that either social networking changes these brain regions, or that people born with these kinds of brains behave differently on websites like Facebook.

In August, Baroness Susan Greenfield, former director of the Royal Institution, made the controversial suggestion that greater use of digital technology might be responsible for increases in the number of people diagnosed with autistic spectrum disorders. The researchers said their work did not directly answer such questions but helped show how future studies could be designed to do so.

"A key question for debate in contemporary societies with online social networks is do people use them in the same way or are they enabling a completely different type of communication and interaction that was never before possible?" said Rees. "People get worried about whether that is in some way affecting or changing our brains or the ways we interact with the world."

He said it was impossible to tell whether the findings meant some people's brains are hard-wired for social networking or whether having a large number of friends on Facebook changes brain structure. "What we're attempting to do is get an empirical handle using the types of data we can generate to try and start that process rolling."

Prof Rees added that future brain scan studies looking at changes to brain structures over time might help unravel whether the brain changes were a cause or effect of having more online social links.

His team carried out magnetic resonance imaging (MRI) brain scans on 165 volunteers who also answered questions about how many Facebook and "real-world" friends they had. They identified three brain regions in which grey matter density was greatest in those with the most Facebook friends, but was not linked to the number of real-world friends they had: the superior temporal sulcus and the middle temporal gyrus, which have previously been associated with the ability to perceive social cues from facial expressions, and the entorhinal cortex, which is linked to memory for things like faces and names.

The density of grey matter in another brain region, the amygdala, correlated with numbers of both real-world and Facebook acquaintances.

Comparing different primate species, researchers have previously demonstrated a correlation between the volume of the neocortex, the part of the brain largely responsible for higher functions like language and thought, and social group size.

Anthropologist Prof Robin Dunbar, of the University of Oxford, has proposed that the number of people with whom humans can maintain stable relationships is limited by the size of our neocortex to an average of around 150. The concept later became known as "Dunbar's number".

He recently led research showing correlations between the size of real-world social groups and the density of grey matter in similar brain regions to those identified in the new study.

"It has been demonstrated that across primate species there is a relationship between neocortex volume generally and frontal neocortex volume in particular and social group size," said Prof Dunbar. "This work and our study are some of the first attempts to show this holds within species as well as between species.

"The interesting question left unanswered is whether this is set in stone and those bits of your brain are hard-wired and determined by your genes, or whether if you bring people up in the right kind of social environment, those bits of the brain grow and therefore the number of people they can maintain as friends in adulthood increases."